@Dorsa , I have enabled the route (Manufacturing and MTO) for Product A and B.Here On hand quantity of product A and B is Zero . Next I Created a manufacturing order for product X.Now system will create a 2 child MO for product A and B separately..

There are two options:

Use Reorder point for the components.  You can set min qty = 0 if you do not want to have stock on hand without requirement.

The dayli schedule job will create manufacturing orders for the components to cover the complete need, if you set Auto in the reorder point.  If you set manual in the reorder point, you must go to Inventory->Operations->Replenishment to create orders according to the requirement. 

The other option is to activate route MTO (Make to Order), and set that route on the components. The result will be that for each order for the finish product, you will get a Child order to deliver the component to the finished product.